Room temperature semiconductor gamma ray detectors have been in use for many years and offer several advantages over other technologies employed in gamma ray detection. Semiconductor detectors include crystals that have good resolution and are robust against temperature change and vibration. Cadmium telluride (CdTe), mercuric iodide (HgI2) and cadmium zinc telluride (CdZnTe) are preferred room temperature semiconductor detector materials that are routinely used due to their ability to operate at room temperature and their inherent high efficiency.
Conventional gamma ray detectors can include a planar crystal having conducting surfaces on opposing faces of the crystal. A bias voltage is applied across the conducting surfaces of the crystal to attract charge carriers, including electron charge carriers and hole charge carriers, that are released upon interaction with a high energy photon incident on the crystal. The electrons move toward a first conducting surface on a first face of the crystal and the holes move toward a second conducting surface on a second face of the crystal.
The signal induced on contacts at the conductive surfaces of the detector crystal is a product of the amount of charge released and its travel distance, the signal being the sum of the signals resulting from the electron travel to the faces of the crystal. The electron charge carriers produce a signal that accurately represents the energy of the incident photon as a function of the depth of interaction, whereas the hole charge carriers are slower moving and have lifetimes that may be short with respect to the travel time across the crystal, so that the contribution from the hole charge carriers may have a deficit. In a simple planar detector, a bias voltage applied across the conducting faces of the detector is normally set so that photons interacting at the front face of the detector will result in the electrons traveling the longer distance through the crystal, with the holes traveling the shorter distance through the crystal to the front contact. If the photon interacts near the front surface of the crystal, then the signal will be dominated by the electron signal, and will accurately represent the energy of the photon. However, when photon interactions occur at greater depths in the crystal, the hole charge carriers become more significant, resulting in a signal with a slower rise time and a lower amplitude. As a result, the photons that interact deeper in the crystal produce an undesirable “hole tailing” effect on the low energy side of the photopeaks. Low energy photons are mostly stopped at the surface of the crystal where they generate pulses with a full amplitude, but higher energy photons that penetrate further into the detector generate pulses having an amplitude deficit, the deficit being caused by poor hole mobility in the crystal. More specifically, hole tailing is caused by an asymmetry in the transport properties of the electron and hole charge carriers; in particular, a high number of holes may become trapped during movement toward the negative electrode, which reduces the collection efficiency and spectral resolution of the detector.
Several techniques have been developed to address and overcome the issue of hole tailing. However, these conventional techniques require complex contact structures such as pixelation or co-planar grids in order to reduce the effect of the hole charge carriers on the detected signal. For example, electron-only detection methods rely on a complicated electrode geometry to produce a detector that is sensitive to charge carrier movement only as it approaches the collection anode or positive electrode at the first face of the crystal. In one approach, a pixelated detector is used in gamma cameras. See H. H. Barrett, J. D. Eskin, H. B. Barber, “Charge Transport in Arrays of Semiconductor Gamma-Ray Detectors,” Phys. Rev. Lett., vol. 75, no. 1, pp. 156-159 (1995), referred to as “Barrett, et al.,” incorporated herein in its entirety by reference. In another approach, electron-only geometries including a coplanar grid are used. See P. N. Luke, “Unipolar Charge Sensing with Coplanar Electrodes—Application to Semiconductor Detectors,” IEEE Trans. Nuc. Sci., vol. 42, no. 4, pp. 207-213, (1995), referred to as “Luke,” incorporated herein in its entirety by reference. In another approach, a drift strip is used. See M. A. J. van Pamelen, C. Budtz-Jorgensen, I. Kuvvetli, “Development of CdZnTe X-Ray Detectors at DSRI,” Nucl. Instr. and Meth. A 439, pp. 625-633 (2000), referred to as “van Pamelen et al.,” incorporated herein in its entirety by reference. However, the abovementioned approaches include a single charge carrier detector having an increased cost due to added electronics and other factors. Pixelated detectors, such as those disclosed in Barrett, et al., incorporated by reference above, require a signal channel per pixel, and coplanar grid detectors, such as those disclosed in Luke, incorporated by reference above, require three signal channels. Further, the electrodes of conventional detectors are also more expensive to produce than planar electrodes due to the masking required, wherein the masking requires extra steps in order to create high resolution electrode patterns on the crystal.
Other approaches in improving the resolution of the spectrum involve accepting only the photons that interact near the surface of the detector. One such method, referred to as Pulse Shape Discrimination, is described in L. T. Jones and P. B. Woolam, “Resolution Improvement in CdTe Gamma Detectors Using Pulse-Shape Descrimination,” Nucl. Instr. and Meth. Vol. 124, pp. 591-595 (1975), referred to as “Jones et al.” incorporated herein in its entirety by reference. This method can produce good resolution at the expense of reduced collection efficiency. However, only the top layer of the crystal is used for photon collection, and photons that interact within the bulk of the crystal, i.e., at depths below that of the top layer of the crystal, are ignored. There is also an added cost in circuitry required to distinguish between the pulses. Often the rise time of the each pulse is measured to determine where the photon interaction occurred in the crystal, and a pulse rejection circuit is required.
Another approach is to correct for the amplitude deficit caused by the hole charge carriers. Whited (U.S. Pat. No. 4,253,023), incorporated herein in its entirety by reference, discloses a method that uses two signal channels with two time constants to separate out the contributions from the electrons and the holes and to correct for the charge deficit of the hole carriers. Saitou (U.S. Pat. No. 4,893,018), incorporated herein in its entirety by reference, discloses circuitry that detects the amplitude and the rise time, then produces a depth signal, and generates a correction signal. Verger, et al. (U.S. Pat. Nos. 5,854,489, 6,420,710), each incorporated herein in its entirety by reference, disclose methods that rely upon measuring the rise time and the amplitude of each pulse separately and computing a correction factor. However, the approaches described above require additional signal channels to measure the depth of interaction and calculate a correction value that is applied to the signal.